1. Field of the Invention
The teachings herein provide techniques for determining properties of various hydrocarbon constituents in a mixture, wherein the mixture is surveyed using a nuclear magnetic resonance (NMR) instrument.
2. Description of the Related Art
A downhole fluid characterization method, such as one implemented by the Reservoir Characterization Instrument (RCI) (produced by Baker Hughes, Incorporated of Houston, Tex.) is used to determine light hydrocarbon types in a formation. This is completed by measuring pressure gradients in a reservoir section. Such measurements treat the fluid as a bulk fluid despite the fact that the fluid may contain multiple components. For example, the fluid may include mixtures of hydrocarbons or mixtures of hydrocarbons and non-hydrocarbon fluids. Thus, classification of hydrocarbon mixtures, such as those having a high gas-to-oil-ratio (GOR) or a low GOR as well as gas condensates can only be inferred qualitatively.
The most commonly inferred parameters for characterizing crude oil in the petroleum industry is either viscosity or fluid density. Both parameters are bulk fluid properties that do not provide for discrimination of compositions of the underlying fluids. In other words, from a measured viscosity or density value alone, one does not have sufficient information to determine whether the fluid generally includes only one type of hydrocarbon molecule, or a mixture of more than one type of hydrocarbon molecules.
On the other hand, use of NMR provides for molecular level characterization of a fluid. That is, NMR may be used to determine information regarding molecular-level interactions among the various molecules. Therefore, a bulk fluid having a hydrocarbon of a single molecular type may have a different NMR response than another bulk fluid that is a mixture having multiple types of hydrocarbon molecules. This is the case even where viscosity, η, of the two bulk fluids is similar or identical. Other techniques for performing microscopic analyses are known. However, these have certain limitations.
For example, although optical technology can in principle provide information regarding individual fluid components in hydrocarbon mixtures, use of optical technology has limitations when optical response of the various mixture components overlap.
Lo et al have studied various aspects of mixtures of hydrocarbons. The mixtures formed are primarily binary (i.e., the mixtures include a first molecule of interest and a second molecule of interest). The studies of Lo et al. have considered gas and oil mixtures as well as oil and oil mixtures. The studies provide that NMR relaxation time or diffusivity distributions from the mixtures may exhibit bimodal distribution patterns. The patterns depend on how close respective relaxation times or diffusivities are, as well as the relative amounts of the individual components in the mixtures.
Other work has been done in this area. For example, in order to correlate NMR response arising from mixtures of hydrocarbon molecules to the commonly used viscosity estimate, Freedman et al introduced a concept of a constituent viscosity model (SPE Journal, December 2001, pp. 452-464, U.S. Pat. No. 6,859,032). Lo et al. have also a mixing rule for NMR based diffusivity and relaxation times in hydrocarbon mixtures (SPEJ, March 2002, pp. 24-34).
Therefore, what is needed is a technique for estimating properties of hydrocarbons in mixtures. Preferably, the technique provides for use of multiple NMR responses to provide the estimating for a variety of mixtures.